The invention relates to a process and a device for converting wet biomass, meaning biomass having a water content of at least 50%, into gaseous products.
For generating energy from wet biomass through thermolysis or thermal decomposition, supercritical water is often used at temperatures and pressures exceeding the critical point of the water (22.1 MPa and 375° C.). Whereas organic substances are dissolved easily in supercritical water, the inorganic salts precipitate out and can result in clogging up the reactor.
A lean reactor is known from the document by N. Boukis, U. Galla, V. Diem, E. Dinjus, entitled Biomass gasification in supercritical water: First results of the pilot plant, published in: A. V. Bridgewater, D. G. B. Boocock, Science in Thermal and Chemical Biomass Conversion, cplpress, Volume 2, p. 975, 2006. Salts from the biomass which do not dissolve in the supercritical water collect on the bottom of this lean reactor. Since the precipitated out salts have a sticky to grainy consistency, only the latter can be separated out and only partially through processes such as the sedimentation. The wet biomass until now has been heated to the reaction temperature in an upstream installed heat-exchanger. However, this process is slow and the reaction already starts in the heat exchanger tube, as soon as the necessary temperature is reached.
According to an alternative embodiment, the preheated and concentrated biomass is fed together with the supercritical water into the reactor, which may require in some circumstance an unfavorable ratio between suspended biomass and supercritical heating water. As a result, the heating process is accelerated and the start of the reaction is moved into the reactor, but most of the salts are precipitated out during the operation in an uncontrolled manner and are hard to remove.
U.S. Pat. No. 4,822,497 discloses the use of a pressure vessel as a reactor for the pollutant oxidation in supercritical water. However, the same problems with respect to the salt formation and the precipitating out occur with a pressure vessel as can occur under reactor conditions. To avoid the undesirable and interfering accumulation of salts inside the reactor, the vessel is provided in the bottom region with a water receiver which is kept, with the aid of quenching, at a temperature that is clearly lower than the critical temperature. Under these conditions, the salts from the reaction space above will again dissolve in water. The oxidation in the supercritical water is strongly exothermal and the quenching is therefore energetically acceptable. In contrast, the gasification of a biomass is endothermal and a quenching is energetically not acceptable.
According to Y. Raja, Gasification of waste to produce low-BTU gas by Molten Salt Technique, J. Institution of Engineers India, Volume 70, Part T2, page 15, 1989, high conversion rates are achieved during the biomass gasification in dry molten salts, but a large amount of carbon monoxide is also generated in the process. In addition, a shift converter must be installed downstream to generate hydrogen. The forming of charcoal or coke can furthermore not be prevented completely during the conversion with dry molten salts.
German patent document DE 202 20 307 U1 discloses a plant for processing fusible materials in supercritical water. The plant consists of a cylinder-shaped reactor with pressure pipes for supplying the reactant and for discharging the product. The pipe for discharging the product is embodied as stand pipe that projects from above into the reactor chamber and ends in the lower third of the reactor. A bottom outlet is installed on the bottom end of the reactor, which is located at the narrowest point and is provided with a cooler and a valve arrangement for the (dis-)continuous withdrawal from the bottom.
U.S. Pat. No. 6,878,479 B2 discloses a device for the direct conversion of fuels into electrical energy, wherein electro-chemical cells containing, respectively, one melted electrolyte are arranged in such a way inside a bipolar, tilted configuration that the electrical resistance between the cells becomes minimal.
A process for the thermal decomposition of graphite fibers and composite polymers in an inert atmosphere is known from the published U.S. Patent Application No. 2005/0066573 A1. With this process, a plurality of graphite fibers bound to a carbonized material are initially produced and the carbonized material is subsequently separated from the graphite fibers with the aid of electro-chemical oxidation in molten salt.
European patent document EP 1 686 192 A1 discloses a process for producing mono saccharides or oligo saccharides from a polysaccharide, wherein the polysaccharide is hydrolized at a pressure of 5 to 100 MPa and a temperature of 140° to 300° C. during a hydro-thermal reaction in hot water, to which carbon dioxide was previously added under pressure.
G. Lee, T. Nunoura, Y. Matsumura and K. Yamamoto disclose in Comparison of the Effects of the addition of NaOH on the decomposition of 2-chlorophenol and phenol in supercritical water and under supercritical water oxidation conditions, J. Supercritical Fluids, Volume 24, pp 239-250, 2002, that the influence of NaOH on the decomposition of organic compounds must be taken into consideration when determining the optimum reactor design.
The overview article by D. D. MacDonald and L. B. Kriksunov entitled Probing the chemical and electrochemical properties of the SCWO system, Electrochimica Acta, Volume 47, pp 775-790, 2001, describes the advantages and the problems associated with using supercritical oxidation in water (supercritical water oxidation, SCWO). As compared to the combustion process, the SCWO has the advantages of closed cycles when implementing the reaction and higher decomposition efficiency. However, the latter advantage is achieved at the expense of higher corrosiveness.
A reactor configuration is disclosed by K. Pripopsky, B. Wellig and Ph. R. von Rohr in SCWO of salt containing artificial wastewater using a transpiring wall reactor: Experimental results, J. Supercritical Fluids, Volume 40, pp 246-257, 2007, available through Science Direct since Jul. 7, 2007. This reactor comprises two different, partially permeable wall elements, which makes it possible to avoid the problem of reactor putrefaction and clogging through salts that precipitate out.
In the document by M. Hodes, P. A. Marrone, G. T. Hong, K. A. Smith and J. W. Tester, entitled Salt precipitation and scale control in supercritical water oxidation, Part A: Fundamentals and research, J. Supercritical Fluids, Volume 29, pp 265-288, 2004, the authors describe the principles of the salt precipitation, the scaling at higher temperatures and pressures, phase diagrams for the salt-water-system, and the resulting phenomena.
The influence of potassium alkalis and sodium alkalis on the de-chlorination of o-chlorophenol in supercritical water is described by Z. Sun, F. Takahashi, Y. Odaka, K. Fukushi, Y. Oshima and K. Yamamoto in Effects of potassium alkalis and sodium alkalis on the dechlorination of o-chlorophenol in supercritical water, Chemosphere, Volume 66, pp 151-157, 2007, available online since Jun. 29, 2006.
M. D. Bermejo, A. Martin, L. J. Florusse, C. J. Peters and M. J. Cocero in The influence of Na2SO4 on the CO2 solubility in water at high pressure, Fluid Phase Equilibria, Volume 238, pp 220-228, 2005, describe the effective decomposition of waste material with the example of Na2SO4 in supercritical water. In the process, decomposition rates of more than 99% were observed for residence times of less than 1 minute.
From M. D. Bermejo, A. Martin, L. J. Florusse, C. J. Peters and M. J. Cocero in Bubble points of the systems isopropanol-water, isopropanol-water-sodium acetate and isopropanol-water-sodium oleate at high pressure, Fluid Phase Equilibria, Volume 244, pp 78-85, 2006, it is known that the oxidation in supercritical water represents an effective, high yield technique for the decomposition of organic waste material. As soon as a sufficient number of cations are present, the existing hetero-atoms are precipitated out in the form of salts and can eventually be recovered.